It is generally known that power plants, process plants, such as refineries, chemical plants or pulp and paper plants, consist of numerous process control loops connected together to produce various products and to control fluid flow. Flow control valves are disposed in the process control loop and typically use a control element such as a valve plug, a valve disc, a globe, or other suitable control element, in order to manipulate a fluid flowing through the system, such as gas, steam, water, or a chemical compound.
It is generally understood that various control valve configurations may be specifically applicable for certain applications. For example, when a quick-opening valve with a narrow control range is suitable, a rotary control valve, such as a butterfly valve, may be used. In any configuration, such control valves are generally coupled to a control device such as an actuator, which controls the exact opening amount of the control valve in response to a control signal. In each case, the valve stem extends into the valve body and is connected to the control element.
In order to provide axial support and to prevent or minimize leakage past the valve stem, control valves typically employ valve packing around the valve stem. For example, FIGS. 1 and 2 illustrate an exemplary rotary control valve 10 of the type commonly employed in process control systems, specifically a quarter turn on/off floating ball valve. The control valve 10 includes a valve body 12, a valve inlet 14, a valve outlet 16, and a flow path 18 that extends between the inlet 14 and the outlet 16. The flow path 18 includes a control passage 20 and a control element 22 is movably disposed in the control passage 20. In the example of FIGS. 1 and 2, the control element 22 in the control valve 10 takes the form of a rotary control element and may be, for example, a valve disc, a globe or ball, or any other form of a rotating control element. In the example of FIGS. 1 and 2, the control element 22 is a floating ball connected to a valve stem 24 which, as outlined above, is a rotary stem. The valve stem 24 is operatively coupled to an actuator 26 by an actuator shaft 28, and the actuator 26 may be any kind of suitable actuator of the types commonly employed in the art. The control element 22 is positioned such that the floating ball is disposed in the control passage 20. Using the actuator 26, the position of the control element 22 within the control passage 20 may be determined, thereby controlling the amount of fluid flow through the control passage 20. The floating ball valve stem 24 may be supported by a shaft adapter 30 and a side-mounted bracket 32 or yoke that operatively attaches the actuator 26 and the valve body 12.
The control valve 10 includes a valve packing 34 which is disposed within the valve body 12. The control valve 10 includes a packing gland 38 that surrounds a portion of the valve stem 24 and includes a lower portion 40 that extends into a position to apply a compressive force to the packing 34. The packing gland 38 may be suitably bolted via fasteners 35 to the valve body 12 (either directly to the valve body 12, or to a bonnet, should the valve have a bonnet).
In a quarter turn on/off floating ball valve, like the control valve 10 illustrated in FIGS. 1 and 2, the valve packing 34 provides minimal axial support to the valve stem 24 when the valve stem 24 is subject to excess side load, axial load, or bending moment. The valve stem 24 may become misaligned with the actuator shaft 28, causing the valve stem 24 to contact the valve body 12. In these instances, the valve 10 requires considerably more torque to operate under normal conditions and galling may occur. Problems resulting from galling include, but are not limited to, diminished life span of valve components and valve failure.